get a 98 boot floppy, if you dont have one get one at
http://www.bootdisk.com/
boot from the floppy, at the
A:\
prompt type
A:\FDISK
hit enter, veiw your partitions and see what all you have, then delete all that apply, primary dos, extended dos, and non-dos(ntfs show up as non-dos)
now boot from the xp cd, if you have a downloaded copy that wont boot the system download the 6 disk set from the above link that will boot the machine and install xp from the cd you have it on. once the installation starts you will be prompted what to do on creating partition(s) and formatiing and then through the install phase.